FBI agents showed up at Steve Bannon’s Washington home last week intent on serving him with a subpoena to appear before a grand jury investigating possible ties between President Donald Trump's campaign and Russia, according to a source familiar with the proceedings.

The agents were unaware at the time that Bannon had retained Washington lawyer William Burck just hours earlier, according to two people familiar with the events that took place on Jan. 9. Once redirected, the agents sent the order to Burck, who is also representing two other witnesses in the probe being led by special counsel Robert Mueller, a former director of the FBI.

Bannon, who served as President Donald Trump’s chief strategist until he departed the White House in August, could end up being interviewed by Mueller’s team before the end of the month, according to one source who agreed to discuss the matter on the condition of anonymity.

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Donald Trump and Steve Bannon

US President Donald Trump (L) congratulates Senior Counselor to the President Stephen Bannon during the swearing-in of senior staff in the East Room of the White House on January 22, 2017 in Washington, DC.

(MANDEL NGAN/AFP/Getty Images)

U.S. President Donald Trump (L-R), is joined by Chief of Staff Reince Priebus, Vice President Mike Pence, senior advisor Steve Bannon, Communications Director Sean Spicer and National Security Advisor Michael Flynn, as he speaks by phone with Russia's President Vladimir Putin in the Oval Office at the White House in Washington, U.S., January 28, 2017. Jonathan Ernst: "Very early in the Trump administration, weekends were as busy as weekdays. On Trump's second Saturday the official schedule said he would be making private phone calls to a number of world leaders including Russia's Vladimir Putin. I arrived early and, before sitting down at my desk walked up to Press Secretary Sean Spicer's office. He, too, was just taking his coat off. I gingerly made the suggestion that previous administrations had sometimes allowed photos of such phone calls through the Oval Office windows on the colonnade. To my mild shock, he didn't even think about it twice. "We'll do it!" he said. In truth, I really only expected the Putin call, but we were outside the windows multiple times throughout the day as the calls went on."

(REUTERS/Jonathan Ernst)

U.S. President Donald Trump talks to chief strategist Steve Bannon during a swearing in ceremony for senior staff at the White House in Washington, U.S. January 22, 2017.

(REUTERS/Carlos Barria)

Trump advisers Steve Bannon (L) and Jared Kushner (R) listen as U.S. President Donald Trump meets with members of his Cabinet at the White House in Washington, U.S., June 12, 2017.

(REUTERS/Kevin Lamarque)

Republican U.S. presidential nominee Donald Trump (C) and campaign CEO Steve Bannon (R) listen to National Park Service Interpretive Park Ranger Caitlin Kostic (2nd R) on a brief visit to Gettysburg National Military Park in Gettysburg, Pennsylvania, U.S. October 22, 2016.

(REUTERS/Jonathan Ernst)

U.S. President Donald Trump (L-R), joined by Chief of Staff Reince Priebus, Vice President Mike Pence, senior advisor Steve Bannon, Communications Director Sean Spicer and National Security Advisor Michael Flynn, speaks by phone with Russia's President Vladimir Putin in the Oval Office at the White House in Washington, U.S. January 28, 2017.

(REUTERS/Jonathan Ernst)

U.S. President Donald Trump signs a memorandum to security services directing them to defeat the Islamic State in the Oval Office at the White House in Washington, U.S. January 28, 2017. Pictured with him are White House senior advisor Steve Bannon (L-R), National Economic Council Director Gary Cohn, Vice President Mike Pence, Deputy National Security Advisor K. T. McFarland, National Security Advisor Michael Flynn, National Security Council Chief of Staff Keith Kellogg and senior advisor Kellyanne Conway.

(REUTERS/Jonathan Ernst)

Trump advisor Steve Bannon (L) watches as US President Donald Trump greets Elon Musk, SpaceX and Tesla CEO, before a policy and strategy forum with executives in the State Dining Room of the White House February 3, 2017 in Washington, DC.

(BRENDAN SMIALOWSKI/AFP/Getty Images)

Senior Advisor Jared Kusher, White House Chief Strategist Steve Bannon and President Donald Trump arrive at the start of a meeting with Senate and House legislators, in the Roosevelt Room at the White House, February 2, 2017 in Washington, DC. Lawmakers included in the meeting were Sen. Orrin Hatch (R-UT), Rep. Kevin Brady (R-TX), Sen. Ron Wyden (D-OR) and Rep. Richard Neal (D-MA).

(Photo by Drew Angerer/Getty Images)

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The subpoena compels Bannon to testify before a grand jury, skipping the voluntary interview with Mueller's team that many in Trump's orbit have elected to take. But Mueller may still leave open the option for an interview in lieu of grand jury testimony. Bannon is likely to accept such an option if it is made available, according to a source close to Bannon.

Three people familiar with the special counsel’s investigation suggested Mueller moved to subpoena Bannon, rather than ask him to voluntarily appear for questioning, in order to thwart any potential attempt by the White House to pressure Bannon into refusing to cooperate.

Bannon revealed he’d been subpoenaed by the grand jury when he met with lawmakers Tuesday behind closed doors on Capitol Hill, where he was questioned for more than 10 hours.

During the course of questioning, Bannon told members of the House Intelligence committee that the White House had instructed him not to answer questions related to his tenure in the administration and the transition period between the election and inauguration, citing a desire to exert executive privilege. The committee responded by issuing a separate subpoena to compel his responses to lawmakers.

Bannon is expected to be more forthcoming with Mueller’s team. “He’ll answer any questions” Mueller wants, one source close to Bannon told NBC News.

Bannon, who headed the right-wing conservative website Breitbart, was instrumental in Trump’s election and a key figure in the White House. But his abrupt departure from Breitbart last week came amid tensions between the two men that exploded in public after the publication of “Fire and Fury,” a book by Michael Wolff which extensively quoted Bannon criticizing Trump and his family.

In the book, Bannon suggests that Trump knew of, and possibly even met with Russian agents who had shopped potential dirt on his political rival to his son, Donald Trump Jr., and to his campaign chairman Paul Manafort. The White House has denied that Trump met with any of the Russian participants at that meeting inside Trump Tower in June 2016.

According to the book Bannon called the meeting, which also included Trump's son-in-law and senior adviser Jared Kushner, "treasonous" and "unpatriotic."

Rep. Mike Conway, R-Texas, who leads the House Intelligence Committee, said the committee’s subpoena remains in effect and described the interview as in “recess.” It was not clear whether Bannon would appear again before the committee, though Rep. Eric Swalwall, D-Calif., indicated late Tuesday that he was hopeful Bannon would return Thursday.

“As with all congressional inquiries touching upon the White House, Congress must consult with the White House prior to obtaining confidential material,” Press Secretary Sarah Huckabee Sanders said. "We have been fully cooperative with these ongoing investigations, and encourage the committees to work with us to find an appropriate accommodation in order to ensure Congress obtains information necessary to its legitimate interests.”

Burck, who is representing Bannon before the special counsel and the congressional investigations, is also representing White House Counsel Don McGahn and Trump’s former chief of staff, Reince Priebus, both of who have met with Mueller late last year.